We propose to characterize a class of ribonucleoprotein complexes (RNP) which is known to occur in mammalian nuclei. This class of RNP was first detected by clinical investigators who found that the sera of humans affected by certain systemic rheumatic diseases (autoimmune diseases) contain antibodies to nuclear RNA-protein complexes. Using antibodies from human anti-RNP sera, we have isolated and partially characterized the RNP antigen from rat liver nuclei. Its RNA moiety is composed of 6 percent by mass polyadenosine (poly(A)) and contains sequences homologous to mRNA. The proetin moiety has also been partially characterized. We have found that the RNP antigen also occurs in dipteran cells, specifically in the salivary gland cells of Drosophila melanogaster larvae. We propose to characterize the RNA moiety of the antigen by DNA-RNA hybridization, to determine the extent to which it is homologout to mRNA. Three of the antigen proteins, which have been isolated, will be characterized in detail by biochemical methods. By indirect immunofluorescence of Drosophila larval salivary gland squashes reacted with anti-RNP antibodies, we have observed that the antigen is localized on the chromosomes and is concentrated in a few of the chromosome bands. The relation between the number of antigenic bands on the chromosomes and the number of different mRNA-homologous RNA species in the antigen will be investigated. The RNP antigen-antibody system will serve as a model for investigating the specificity of autoantibodies and for understanding the relation between the occurrence of antibodies and the associated disease. This project should, therefore, have considerable clinical significance.